In recent years, in cellular mobile communication systems, transmission of large volumes of data such as still image data and moving-image data as well as audio data has been generalized with accompanying the increase of multimedia information. Techniques for realizing a high-transmission rate using a high-frequency wireless band have been actively studied to realize the transmission of large volumes of data.
When the high-frequency wireless band is used, however, a high-transmission rate is expected over a short distance, but attenuation increases as the transmission distance increases. Accordingly, when a mobile communication system using a high-frequency wireless band is actually operated, a coverage area of a wireless communication base station apparatus (hereinafter, simply referred to as a base station) is reduced. Therefore, it is necessary to install more base stations. Since the installation cost of base stations is considerable, techniques are required to realize communication services using a high-frequency wireless band while reducing the number of base stations.
To meet this demand, a relay transmission technique for installing a wireless communication relay station apparatus 20 (hereinafter, simply referred to as a relay station) between a base station 10 and a wireless communication mobile station apparatus 30 (hereinafter, simply referred to as a mobile station) and performing communication between the base station 10 and the mobile station 30 via the relay station 20 has been studied to increase the coverage area of each base station, as in a wireless relay system shown in FIG. 13. When a relay technique is used, even a terminal that ma not directly communicate with the base station can perform communication via the relay station 20. Further, a wireless communication mobile station apparatus 31 is connected directly to the base station 10.
[Description of TD Relay]
In a TD relay (also referred to as a half duplex relay), transmission from the base station 10 to the relay station 20 and transmission from the relay station 20 to the mobile station 30 are performed in a time division manner. FIG. 14 illustrates the TD relay in a downlink (DL) and FIG. 15 illustrates the TD relay in an uplink (UL). In FIGS. 14 and 15, the vertical axis indicates a time domain.
In the downlink shown in FIG. 14, transmission from the relay station 20 to the mobile station 30 is performed with an access link in subframe #2 and communication from the base station 10 to the relay station 20 is performed with a backhaul link in subframe #3. Then, the transmission from the relay station 20 to the mobile station 30 is performed again in subframe #4. Likewise, in the uplink shown in FIG. 15, transmission from the mobile station 30 to the relay station 20 is performed with the access link in subframe #2 and communication from the relay station 20 to the base station 10 is performed with the backhaul link in subframe #3. Then, the communication from the mobile station 30 to the relay station 20 is performed again in subframe #4.
As shown in FIGS. 14 and 15, the transmission time of the relay station 20 and a reception time of the relay station 20 can be divided by dividing the communication of the backhaul link of the relay station 20 (Relay) and the communication of the access link of the relay station 20 (Relay) on the time domain. Accordingly, the relay station 20 can perform relaying without an influence of loop between a transmission antenna and a reception antenna.
In LTE, data (referred to as UL data) of an uplink (hereinafter, referred to as UL) is allocated with a control signal called a UL grant. In an FDD system, it is regulated that UL data is allocated after four subframes of the UL grant transmitted in the downlink (hereinafter, referred to as DL). That is, the UL grant does not have information describing when the UL data is allocated, but it is allocated after four subframes at all times.
In the FDD system for LTE, it is regulated that UL data is transmitted in a UL subframe, and then ACK/NACK for the UL data is also transmitted in a DL subframe subsequent by four subframes. Further, it is regulated that ACK/NACK for DL data is also transmitted in a UL subframe subsequent to DL data by four subframes.
In LTE, as described above, the UL data is transmitted in the UL subframe after four subframes of the UL grant and ACK/NACK for the UL data is transmitted in the DL subframe after four subframes of the UL data. That is, it is regulated that the UL data is transmitted at 8TTI (Transmission Time Interval). According to this regulation, since it is not necessary to give a notification of a subframe number, overhead can be reduced. Further, “TTI” refers to a unit indicating the interval of a transmission time. Here, a relation of “1TTI=1 subframe” is satisfied.
In LTE, it is regulated that when ACK/NACK for the UL data and the ACK/NACK for the DL data are transmitted using the same UL subframe, the ACK/NACK signal for the DL data is multiplexed to the UL data and is transmitted. This is because it is difficult to simultaneously transmit the ACK/NACK signal for the UL data and the ACK/NACK for the DL data by each carrier since a transmission type of the UL data of LTE is a single carrier.
[Position of Backhaul]
In the TD relay, the relay station 20 receives a control signal from the base station 10 using only a subframe set in backhaul. Thus, the relay station 20 determines a “candidate position of the backhaul subframe” in advance. It is considered that the “candidate position of the backhaul subframe” of the DL is determined by the base station 10 and is noticed to the relay station 20. Whether the “candidate position of the backhaul subframe” of the DL is used in practice in the communication of the backhaul is notified using a control signal destined for the relation station 20 and transmitted from the base station 10
According to the same rule as LTE, the “candidate position of the backhaul subframe” of the UL is regulated as a “subframe after four subframes of a DL subframe” in the FDD system. Therefore, the “backhaul subframe after four subframes of a DL subframe” can be regulated to a “subframe of a backhaul” of the UL. In the UL, whether the “subframe of a backhaul.” of the UL is used in practice in the communication of the backhaul is notified using a UL grant destined for the relation station 20 and transmitted from the base station (see Non Patent Literature 2)